Live in Southern Florida, so we use AC nearly year round. We live in a block house, concrete tile roof, fairly new, well insulated. We do not use a programmable thermostat as we are home most of the time, and when not it is not predictable times.
However… recently had the AC replaced, but it had not completely failed. We got the house down to 75 before they started about 8:30am, and they finished at 6pm and the house was only about 79 degrees. Gave me an idea…
From 1am to 10am (+/-) will set it to 75 and over-cool a bit while the outside air is about 20-25 degrees cooler than daytime. The house has a large heat capacity, and cools and warms slowly. That ought to hold us under 78 until mid-late afternoon, so the unit would not run (or not much) at all during the hottest air temperature time.
In other words, try using it to shift the biggest cooling load into the most efficient time, in the wee hours, and during the least efficient time try not to run it (hardly) at all.
Does this sort of cold-soaking work reasonably? Any guidance as to whether it will be an improvement, or are we making it worse by the cooling below 78, rather than just keeping it at 78 all day.
Best Answer
It's all about thermal mass
You say your house is concrete block and insulated, now where's the insulation? If the insulation is outside the block, then you are using a technique used in passive-solar construction called thermal mass. **, Trying to have a large thermal mass in the house which resists change in temperature. If the insulation is inside the concrete, different deal, then it is your friend in the morning and your enemy at night.
If it can store/ resist temperature change for 12 hours, then this is worth doing.
Otherwise its just yet another excuse people make up when they want to crank the A/C.
The other thing is, your thermal mass may be different than most. But if this worked for most people, the power company would be telling everyone to do it. They want to sell you power, sure, but the hardest thing for them is producing peak power. Anything they can do to move power consumption from the peak to the valleys (night/morning) they're all about.
Based on what you say, I don't think a 3 degree swing will matter much. The stored energy is degree difference x mass, and a small temperature difference doesn't give you much storage.
There are better ways to do this
First, the best way to avoid interchanging with 120 degree air is interchange with 50 degree groundwater. If you have groundwater in your area that is near enough the surface to be worth pumping. You have to buy a condenser made for this, of course. It works particulary well with heat pumps, because groundwater has a usable temperature all year, even in cold places. You make yourself 2-5 watts of heat for 1 watt of electricity, the other 1-4 watts come from a very large RTG called planet Earth.
Second, you do the same thing using dirt itself, but the coolant loop must be much, much larger, because it needs to interchange with a lot of earth.
Third, if your house isn't built with a thermal mass, you can get your own thermal mass with a huge insulated tank of water. At night, you use cheap evening electricity to chill the water, by day you use it to interchange heat, as above. Nowhere is it written the tank of water can't have a concrete skirt, a shallow end and a diving board... But it does need to be well insulated.